Photochemical Ignition Studies. I. Laser Ignition of Flowing Premixed Gases

摘要

Gas mixtures of CH4/air, CH4/N2O, C3H8/air, C3H8/N2O, & C2H2/air were ignited above a slot burner by focusing three different laser beams whose wavelengths are 193 nm, 248 nm, & 532 nm. Minimum ignition energies were determined over a wide range of equivalence ratios & over the energy range of 0.1-40 mjoule/pulse for each gas/laser combination. A substantial wavelength dependence of the minimum ignition energy was observed for the ArF (193 nm) and KrF (248 nm) lasers acting on the different gas mixtures & was attributed to molecule-specific multiphoton-induced photochemistry of the various fuel & oxidizer molecules. Here, the necessary radicals &/or ions which are needed to cause ignition to occur are apparently produced in a controlled way. The Nd:YAG second harmonic (532 nm) laser exhibited a much smaller minimum ignition energy range for the various gas mixtures due to a different ignition mechanism involving gas breakdown, i.e., a laser-produced spark. This process appears to be much harder to control with respect to energy deposition than the photochemical one. The most efficient laser-driven ignition system was the one where the ArF (193 nm) laser acted on C2H2/air & laser energies as low as 0.2 mjoule caused ignition. Our results further indicate that there should be a number of ways to improve the efficiency of the photochemical ignition process. Thus, this new type of ignition source appears to possess considerable potential for utility in both practical applications as well as in allowing direct, time-resolved studies of the chemistry of ignition itself, which is an area of considerable current interest.